A comparison of thermo-mechanically-treated and electron-beam-welded strong, ductile medium-entropy alloy: Microstructural evolution and deformation mechanisms

Authors：Peng, Hanlin^*; Hu, Ling^*; Huang, Siming; Li, Liejun; Baker, Ian

Source：Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing , 2023, 882: 145449.

DOI：10.1016/j.msea.2023.145449

• Medium-entropy alloys
• Electron beam welding
• Deformation mechanisms
• Phase transformation
• Microstructure
• Mechanical properties

The microstructure, room-temperature deformation mechanisms, and weldability of the medium-entropy alloy (MEA) NiCoVAl0.2 have been investigated. Defect-free welded joints have been produced with good mechanical properties, indicating excellent weldability. The microstructure consists of f.c.c. grains containing many b.c.c. Ni2VAl (Ni26.1Co33.5V27.8Al12.6) and few a-type precipitates (Ni15.5Co21.3V62.7Al0.5). Decreasing the aging tem-perature from 1100 & DEG;C to 900 & DEG;C, produced a higher area fraction of Ni2VAl precipitates (8 & RARR; 25%), much smaller L21 precipitates (7.2 & RARR; 1.4 & mu;m), finer grains (17.1 & RARR; 2.4 & mu;m), and presence of a-type precipitate, resulting in simultaneous improvements in both the strength and ductility for both the thermo-mechanically treated (TMT) and electron beam welded (EBWed) MEA. The TMT MEA shows an excellent combination of strength (YS-993 MPa, UTS-1478 MPa) and ductility (30.3%), while the EBWed MEA shows slightly lower strength (YS-822 MPa, UTS-1194 MPa) and significantly reduced ductility (12.3%), i.e. a YS, UTS, and ductility of 83%, 81%, and 41%, respectively, of those of the TMT MEA. With increasing strain, both low-angle grain boundaries and geometrically-necessary dislocation (GND) densities increased. The TMT MEA has a higher GND density (1.6 x 1015 m-2) than the 9.1 x 1014 m- 2 of EBWed MEA after strained to fracture. For both the TMT and EBWed MEA aged at 900 & DEG;C, the deformation was accommodated by dislocation slip, dislocation looping around precipitates, and nano-twinning. However, for both the TMT and EBWed MEA aged at 1100 & DEG;C, more abundant deformation twinning and an fcc & RARR;hcp shear transformation appeared, which may originate from a lower stacking fault energy due to differences in the f.c.c. matrix composition.